Resolution of structural evolution and 3D modelling of Co-Cu mineralization in a sheared basement complex, Mwombezhi Dome, Lufilian Arc
Following persistent bench-scale failures and poor bench definition of Chimiwungo South Pit’s eastern and northern highwalls, a structural study was commissioned by Lumwana’s Geotechnical Department in 2016. The study’s aim was to resolve the structural origins of these failures, to construct a robust 3D geotechnical model and to advise on mitigation measures. A comprehensive structural mapping program was undertaken to delineate relevant ductile to brittle structures and fabrics in the Chimiwungo South Pit. This data was integrated with downhole geotechnical, macrostructural and lithological data to resolve tectonostratigraphic horizons, internal ductile and brittle structures and fabrics, and their cross-cutting relationships, in a fully-constrained 3D structural-lithological model in Leapfrog.
Tectonostratigraphic domains included the main Ore Schist and its hangingwall and footwall units. Three distinct sets of tectonic fabrics were resolved. S1 represents relict gneissosities and schistosities induced during peak metamorphic conditions, burial deformation and low-angle fore-thrusting of the Lufilian Orogeny (ca. 580-500 Ma). S2 is characterized by an incipient to well-developed mylonitic fabric, sub-parallel to S1 on a pit and regional scale. At the centimeter to 100m scale, S2 mylonites often cross-cut S1 fabrics at low to moderate angles, as S2 records opposite kinematics due to basement under-thrusting.
S3, which is attributed to the formation of a regional-scale basement extensional detachment during post-orogenic collapse of the Lufilian Belt (ca. <500 Ma), is characterized by an incipient to well-developed, fissile, mylonitic fabric that also hosts Cu-sulphide Co mineralization. S3 cross-cuts S1-2, resulting in pit-scale boudinage and lozenge formation in hangingwall and footwall tectonostratigraphic domains. The structural position and rotation of these S3 lozenges and their internal, relict S1-2 geometry, particularly in the hangingwall which constitutes most of Chimiwungo’s current highwalls, dictates the overall geotechnical design. The volume is further offset by late-tectonic D4 (post-Lufilian) brittle faults which, on a pit-scale, are geotechnically very weak and act as release planes for unfavorably-dipping mylonitic structures (S1-3). These faults juxtapose more ductile domains that contain opposing or different fabric orientations, which may locally dip out of the face.
Mapping was subsequently expanded to Chimiwungo West and North Pits from 2017. The initial scope of work was broadened to include refined lithological modelling of tectonostratigraphic domains and the development of form surfaces that represent relevant S1-3 fabrics. Angle-contoured apparent dip maps of S1-3 fabrics were compiled to highlight unfavorable geotechnical domains for monitoring during pushbacks and on the final pit design. These form an important input into design sector delineation. Training of Lumwana Geoscientists was undertaken, concurrent with mapping and 3D modelling, to facilitate knowledge transferal and upgrading of Leapfrog modelling skills. These were implemented on-site, as part of basic “on-data” training and off site, in the Leapfrog Masterclass training provided by Tect.
The results of this project were published an international, peer-review publication. A second paper, which is in preparation, discusses the links between the structural inventory and its application to geotechnical modelling.
Pan-African structural evolution of Paleoproterozoic basement gneiss and Cu-Co mineralized shear zones in the Domes Region of the Lufilian Belt, Mwombezhi Dome, Zambia by Corné Koegelenberg, Ian Basson, Henry Sinkala, Hastings Lupapulo and Peter Hornsby, Journal of Structural Geology, 127, 16pp.
(In preparation) Integrated structural and geotechnical modelling of sheared Paleoproterozoic basement complex, Lumwana Mine, Mwombezhi Dome, Zambia by Corné Koegelenberg, Peter Hornsby, Ian Basson, Hastings Lupapulo, Peter Creus, Engineering Geology or similar.
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